As the dimensions of integrated circuits become smaller and smaller, depth of focus (DOF) of the lithographic process employed to pattern them also becomes very small. If DOF of the conventional lithographic scheme approaches the control limit that can be provided by state-of-the-art exposure tools, then DOF-enhancement techniques must be taken into account.
In the literature, there are several ways that can enhance DOF. One is the introduction of alternating phase-shifting mask (Alt-PSM), and the other, off-axis illumination (OAI).
To illustrate how these techniques can enhance DOF, we first consider the imaging process of an exposure tool. Shown in FIG. 1 is a schematic sketch of a double-telecentric projection imaging system with Kohler-type illumination, which ensures that illumination is very uniform on the mask. The spherical wave from the source 11 after the condenser lens 12 becomes the plane wave illuminating the mask 13. The diffracted waves behind the mask are collected by projection lenses 14, 16 to form images on the wafer 17. The pupil 15 defines the highest spatial frequency that can be captured by the imaging system. The mask plane is the Fourier-transform plane of the source plane. The pupil plane, being the Fourier-transform plane of the mask plane, is also the image plane of the source plane. The wafer plane, being the Fourier-transform plane of the pupil plane, is also the image plane of the mask plane.
It can be readily shown that if all the diffraction spots on the pupil plane are of equal distance from the pupil center, then the resulting image on wafer will be independent of z. That is, DOF will be infinite.
We use the imaging of one-dimensional (1-D) line/space as examples. Shown in FIG. 2a is the conventional lithographic scheme, which employs on-axis illumination along with a binary mask (BIM). The DOF of such a lithographic process cannot be very large since the collected diffraction orders, including order 0, order−1, and order 1, to form image on the wafer are not all of equal distance from the pupil center. If we only collect order 0, the image can't be formed since at least two diffraction orders must be collected.
The principal of Alt-PSM is illustrated in FIG. 2b. One of the two clear regions beside a dark feature of mask patterns is phase-shifted by 180 degrees to eliminate diffraction order 0. DOF will be very large since only order −1 and order 1 are collected and the two diffraction orders are of equal distance from the pupil center. The principal of OAI is illustrated in FIG. 2c. Order 0 is shifted from the pupil center by illuminating the mask off-axially. For line/space patterns of a certain pitch, specific illumination can be chosen such that order 0 and order 1 or order 0 and order −1 are of equal distance from the pupil center. Therefore, DOF will be very large.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,096,457, Pierrat shows an off-axis illumination process for a phase shift mask. Rolson (U.S. Pat. No. 6,057,065), Von Bunau (U.S. Pat. No. 5,863,712), and Gortych et al. (U.S. Pat. No. 5,680,588), all show related patents.